It is sometimes advantageous to be able to control not only the reactive power, but also to inject or absorb active power, also denoted real power. The STATCOM can therefore be provided with an energy source on its DC side in order to provide some active power, in addition to the reactive power generated to the network. For example, the real power can be utilized either as a source of reserve power when an energy deficit occurs within the network, or as a control power for managing transients and electromechanical oscillations in the network. The energy source may for example comprise conventional capacitors, super capacitors or electrochemical batteries.
FIG. 1 illustrates a basic STATCOM arrangement, or voltage source converter system, having an active power source on its DC side, wherein the energy source in the illustrated example comprises a number of parallel-connected battery strings 2. Each battery string 2 comprises a number of battery modules and each battery module in turn contains a number of battery cells. The STATCOM 1 is also connected to a power network 3 in a conventional manner via a transformer (not shown) to adapt the converter voltage to the network voltage and to provide galvanic isolation.
Often, when electrochemical batteries are used as the energy source, a high number of battery modules must be connected in series to match the DC voltage of the STATCOM. Moreover, in order to obtain the desired active power and duration of the battery energy storage, a number of battery strings often has to be connected in parallel, as is illustrated in the figure. The STATCOM DC voltage is controlled and all the parallel-connected battery strings are connected to this controlled voltage.
During operation of the battery energy storage, the cells age differently. Further, some of the cells may have failed and in order to continue operation, the broken cell(s) of the battery modules of the battery string will be bypassed. Moreover, some aged battery modules will be replaced by new modules. For batteries with a low internal resistance, a small voltage offset will result in charge redistribution. The performance of the whole battery energy storage will be reduced, as it will be dominated by the battery cell with the lowest performance.
To bypass the failing battery cell(s) is thus done at the expense of reduced charging voltage of the battery string. Another solution is to bypass the whole battery string, but then an even greater capacity redundancy in the form of oversized batteries and additional battery strings would be required to meet capacity requirements at all times. Today, parallel-connected battery strings are thus not run in an optimal fashion.
In view of the above, it would be desirable to provide an improved way of handling failing battery cell(s) and/or battery strings.